1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus in which an image is developed by using a toner or like colored particles, such as an electrostatic copier, a printer, or a facsimile apparatus, and more particularly to a transfer section of such an apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a latent image is formed in the surface of an image carrier by using a laser beam, and the image is then developed by supplying a toner from a development roll. The development system is roughly classified as either the normal development system or the reversal development system according to the charge polarity of the surface of the image carrier and that of the toner. As a result of the development process, the toner is electrostatically attracted to the surface of the image carrier to form a toner image, and a voltage of a polarity which is opposite to the charge polarity of the toner is applied by a transfer charging device of, for example, the roller transfer system using a transfer roller which butts against the image carrier, or the corona transfer system, so that the toner image is transferred to a transfer member.
During the process of transferring the toner image which is formed by the development process, to the transfer member, when the transfer member enters between the image carrier and the transfer charging device, the toner tends to be scattered. It is usually recognized that the transfer efficiency of the corona transfer system is lower than that of the roller transfer system. For the corona transfer system, therefore, a method in which a voltage is applied to a sheet transport guide or the like before the transfer process has been proposed in, for example, JP-A-61-32667, JP-A-10-26888 and JP-A-2000-181322.
For the roller transfer system, a method in which a transfer member is once brought into contact with an image carrier to prevent a toner image from being scattered has been proposed in, for example, JP-A-2-176779. Another method in which a voltage of a polarity that is identical with that, of a toner, i.e., that is opposite to that of the transfer process is applied to a zone where a transfer member does not exist, thereby preventing the toner from adhering to a transfer roller has been proposed in, for example, JP-A-51-9840 and JP-A-3-69978.
In the method in which a voltage is applied to a sheet transport guide or the like before the transfer process in the corona transfer system, a structure for electrically insulating the sheet transport guide from other parts surrounding the guide, and electronic components for controlling the voltage applied to the sheet transport guide must be additionally disposed, and the image forming apparatus itself must be complicatedly configured.
In the method in which a transfer member is brought into contact with an image carrier to prevent a toner attracted to the image carrier from being scattered in the roller transfer system, the transfer member is in contact with the image carrier in a zone previous to the transfer process. Therefore, end faces of the transfer member are smeared with a toner which is electrostatically attracted to, for example, a background area of the image carrier, whereby the print quality is extremely lowered. The method has further various problems such as that, depending on the angle of a contact between the transfer member and the image carrier, or the thickness of the transfer member, the photosensitive surface of the image carrier may be damaged at the contact, and that noise is generated at the contact.
It is an object of the invention to provide an image forming apparatus which is free from the defects of the conventional art, and in which a satisfactory image can be obtained by a simple configuration.
In order to attain the object, the invention is directed to an image forming apparatus comprising: an image carrier which carries a toner image, such as a photosensitive drum; a transfer rotary member which transfers the toner image from the image carrier to a sheet or a like transfer member, such as a transfer roller; a bias power supply which applies a voltage to the transfer rotary member; and a transfer member transporting section, such as a transport roller or a transport guide which transports and guides the transfer member toward a nip area that is formed by the image carrier and the transfer rotary member.
The invention is characterized in that the transfer member transporting section is disposed to bring a tip end of the transfer member into contact with a peripheral face of the transfer rotary member before the tip end of the transfer member enters the nip area, and
the bias power supply has a polarity inverting function of: during a period from a timing when the tip end of the transfer member is brought into contact with the transfer rotary member by the transfer member transporting section, to a timing when the tip end of the transfer member enters the nip area, applying a voltage of a polarity which is identical with a polarity of a toner, to the transfer rotary member; and, after the tip end of the transfer member enters the nip area, applying a voltage of a polarity which is opposite to the polarity of the toner, to the transfer rotary member.
The invention is characterized in that, in the above configuration, an elastic layer such as a sponge layer or a layer of electrically conductive rubber is formed in an outer peripheral portion of the transfer rotary member.
According to the thus configured apparatus of the invention, smearing of the transfer member caused by contact between the image carrier and the transfer roller can be prevented from occurring, and a high-quality image can be obtained.